This application claims priority of German Application No. 100 43 269.7, filed Aug. 29, 2000, the complete disclosure of which is hereby incorporated by reference.
a) Field of the Invention
The invention is directed to a diode-pumped laser amplifier with at least one laser-active solid-state medium which is permeated by the mode matched beams of a pump radiation and laser radiation to be amplified and in which a thermal lens forms because of the irradiating pump radiation whose lens strength is different in orthogonal planes.
b) Description of the Related Art
It is known that in the field of solid-state lasers fundamental mode oscillators can only be scaled up to a certain output power because of the optothermal properties of the laser-active media that are being used. For the generation of a high power output, oscillators are therefore often combined with laser amplifiers arranged behind them in the beam path with which the high quality of the beam of the oscillator radiation can be maintained.
Such an oscillator-amplifier arrangement also has the advantage of making it possible to intentionally run an oscillator at relatively low power. In the case of a mode locked short pulse laser, this means that a saturable semiconductor absorber arranged inside a resonator can thus be protected from overloading.
Known arrangements, like, for example, the one according to U.S. Pat. No. 5,237,584, have the disadvantage of having a high adjusting sensitivity. The oscillator output radiation is supplied mode matched to the multi-stage amplifier. The amplifier contains diode-pumped laser crystals with folding mirrors arranged opposite. To compensate for the strong thermal lenses forming in the crystals, a highly precise tuning of the radii of curvature of the mirrors and of their distances to the laser crystals is necessary; otherwise, a stable optical system can not be guaranteed. One serious disadvantage of these serial set-ups is the fact that individual component variations must be adjusted for over and over again. Small changes, like, for example, variations of the diode parameters caused by aging or other things, as well as component variations of the crystals, already lead to a system breakdown because of the very narrow tolerances.
The named disadvantages are even more marked in a laser system according to U.S. Pat. No. 5,696,786, since intermediate imaging elements are not used here and an opportunity for adapting the beam path with regards to the thermal lenses therefore does not exist. Such laser arrangements typically only have small operating windows when it comes to diode currents.
Alternatively, the folded beam path can also be realized in a crystal (U.S. Pat. No. 5,271,031) by consecutively going through different pumped areas of this crystal a number of times. This arrangement also has the disadvantages of U.S. Pat. No. 5,696,786.
Finally, from a longitudinally pumped solid-state laser arrangement according to DE 195 21 943, the fact is known that convex toric or cylindrical lenses can be generated spontaneously and uncontrollably by an elongated pump spot because of the isothermal field which builds up in the laser crystal and is shaped like relatively flat ellipses.
It is the primary object of the invention to increase the tolerance of the amplifier arrangement towards variations of the input parameters considerably so that fine-tuning to guarantee the stability of the amplifier is no longer necessary.
This object is met by a diode-pumped laser amplifier with at least one laser-active solid-state medium which is permeated by mode matched rays of pump radiation and laser radiation to be amplified, and in which a thermal lens is formed because of the irradiating pump radiation whose lens strength is different in orthogonal planes, and by directing the beam of the laser radiation into the laser-active solid-state medium focussed in the plane with a strong thermal lens, with the beam waist that is created lying in the area of the thermal lens.
For the creation of the beam waist, a focussing element like, for example, a cylindrical lens focusing in the plane of the strong thermal lens is used which is arranged in the beam path of the laser radiation at a distance to the location of the thermal lens corresponding to the element""s focal length.
Since the distance to be established between the cylindrical lens and the laser-active medium is not a function of the thermal lens, setting the distance at the nominal focal length once is sufficient. Individual thermal lenses, like those to be found in laser-active solid-state media for amplifiers with multiple amplifier stages, no longer have to be taken into consideration when it comes to establishing a stable system. It has become evident that a thermal lens located in the area of a beam waist does not influence beam propagation. Contrary to prior art solutions where the influence of thermal lens effects on the beam path must be compensated for by highly sensitive adjustments, in the invention the effect of thermal lenses has already been practically removed by the amplifier configuration. The laser beam undergoes a mode matching that is independent in planes with thermal lenses of differing strength. Since by this the amplification, beam quality and beam parameters of the amplifier are also insensitive to variations of the diode parameters, the pump diodes realized as laser diode arrays can be operated in series in the electrical sense and at the same temperature. In contrast to prior art solutions, different thermal lenses caused by different individual diode laser characteristics and spectral properties also lose their effect and therefore do not need to be compensated for with complicated adjustments.
Using the invention, a compact and inexpensive amplifier can be constructed whose number of strictly modular amplifier stages is scalable since the output of each amplifier stagexe2x80x94with the exception of an end stagexe2x80x94forms the input of a subsequent amplifier stage. This provides an amplifier arrangement for which wide production tolerances of the necessary simple standard components are admissible and which still only has a very low variation of the amplifier.
With the arrangement, the radiation of low-power oscillators with a mode locked operating mode can be amplified while maintaining the beam quality defined by the oscillator and determined by the times-diffraction-limit number M2. The construction of a serial amplifier with a high degree of stability and reproducibility has been achieved. Furthermore, the use of the invention avoids having to use price-increasing beam shaping optics for the pump diodes which decrease effectivity and with which normally a rearrangement in the sense of a symmetrization of the beam parameter products for the fast and the slow axis of the diode is done.
Advantageously, at least one laser diode array is used as the pump radiation source for which means for the collimation of the fast axis and means for focusing the diode radiation in the elliptical pump focus for the generation of the thermal lens formed with different strengths in orthogonal planes are provided.
For the improvement of the beam quality of the laser diode array, an additional collimation of the slow axis can also be advantageous.
For setting the polarization, a half-wave phase plate can be provided.
The pump radiation and the laser radiation are directed into the solid-state medium through beam passage surfaces lying opposite to each other. Since the beam passage surface for the pump radiation is highly reflective for the wavelength of the laser radiation, the laser radiation after being reflected again passes through the laser-active solid-state medium and the focusing element.
An optical element for the expansion of the laser beam in the plane where the thermal lens is weak arranged on the input side of the amplifier can be realized as a Galilean telescope with cylindrical lenses or as an anamorphotic prism pair.
It is advantageous if at least one image forming element is provided in the beam path of the laser radiation for the purpose of compensating the weak thermal lens in the one plane. This can either be a cylindrical mirror realized as folding mirror or a spherical mirror; alternatively, a convex cylindrical lens with a long focus length or a spherical lens with a long focus length can be arranged in the beam path of the laser beam emerging from the laser-active solid-state medium.
A Nd.YVO4 crystal with a doping level of 0.5% can be used as the laser-active solid-state medium and pumped at a wavelength in the range from 803 nm to 809 nm.
The invention is subsequently to be described in more detail using the schematic drawings.